The myc proto-oncogene functions normally in control of cell proliferation and, when deregulated, is profoundly involved in the genesis of a wide spectrum of tumors. The Myc protein is a bHLHZ class transcription factor which binds DNA through dimerization with the small bHLHZ protein, Max. Myc is but one of several bHLHZ proteins (including the Mad family) which interact with Max and together comprise the Max transcription factor network. Interestingly the members of this regulatory network function as transcriptional activators or repressors and act to buffer or antagonize each others functions. The transcriptional activities of these proteins are mediated by direct recruitment of co-repressors (Mad-Max recruits the mSin3-HDAC co-repressor complex to DNA) or co-activators. This application is directed towards understanding the molecular mechanisms through which specific components of the network function and their biological roles in cell proliferation, growth, differentiation, development, and oncogenesis. The first aim is focused on the role of the mSin3-HDAC repression complex, which is involved in gene repression related to differentiation and signal transduction. The basis for the specific interaction between defined regions of mSin3 with specific transcription factors (such as Mad) will be elucidated and used to define altered specificity mutants. The broad developmental role of mSin3A will be assessed through targeted gene deletion in mice. Aim 2 employs the power of Drosophila to understand more deeply the functions of the network as a whole. Our earlier work demonstrated that Drosophila and vertebrate Myc play direct roles in regulating cell growth (size). We propose to employ endoreplicating larval cells to delineate the key components in the dMyc and dMad growth regulation pathways. This will be done, in part, by characterizing null mutations in dmyc and dmad. We will analyze direct chromatin binding of Max network proteins. In Aim 3 we will use retroviral "gene tagging" in myc transgenic mice with defined genetic backgrounds in order to identify and characterize genes that cooperate with myc during lymphomagenesis. We will identify myc-cooperating genes under conditions of increased sensitivity to apoptosis (i.e. mdm2 hypomorphic mutant mice) or under conditions where myc function is no longer antagonized by Mad proteins (i.e. mad knockout mice). Such genes are expected to define new functional pathways for myc in tumors.